1. Field of the Invention
The present invention is related to the field of telephony using a packet network protocol, and more specifically to devices, softwares and methods for encoding voice data to conceal packet loss.
2. Description of the Related Art
Networks such as the internet are used for telephony, in addition to sending data. Accordingly, voice is encoded into digital data, the data is arranged in packets, and the packets are transmitted to the recipient over a network. Telephony requires that this process happen in real time.
A disadvantage of protocols that permit real time use is that they are unreliable, in that they permit packets to be lost, without retrieving them. When that happens, the voice segments they were carrying are not reconstructed, and the recipient hears annoying gaps in speech. These gaps are perceived as reduced quality of service.
In order to conceal the fact that a packet has been lost, redundancy schemes have been devised. Redundant packets are encoded and transmitted, which repeat aspects of the original data. If a packet is lost, its data is recovered and/or reconstructed from its corresponding redundant packet, which is hopefully not lost. A jitter buffer at the receiving end collects the primary and redundant packets, and then plays them out.
Packets tend to become lost in groups, which are called bursts. Redundant data is sent with a redundant-coding delay from the original data. The delay is chosen to be long enough to avoid large bursts of packet loss and small enough to where the resulting delay at the receiving end is tolerable.
A problem, however, is that the burst length of packet loss episodes fluctuates with time. If the burst length exceeds the chosen delay, the redundant packets will also be lost, and the redundancy scheme will not work. On the other hand, while the burst length is much smaller than the delay, the delay turns out to be unnecessarily long for playout. Accordingly, the manufacturer has to make determinations as to what delay to program into the system. And every such determination will optimize a system for one set of conditions, while rendering it sub-optimum for others.